One of the reasons for utilizing a fluid jet electrostatic applicator (such as the applicator disclosed in U.S. Pat. No. 4,650,694 which is hereby expressly incorporated by reference), is to achieve a fairly tight control over the amount of fluid that is actually applied to the textile in a given treating process (e.g., dyeing). In many conventional textile liquid treatment processes, a considerable amount of excess "add-on" liquid is necessarily applied to the textile. Subsequently, much effort and expense are typically encountered in removing this excess fluid from the textile. For example, some of the excess might be physically squeezed out of the textile (e.g., by passage through opposed rollers), but much of it will have to be evaporated by heated air flows or the like. This not only requires considerable investment of equipment, energy, time and real estate, it also often produces a contaminated flowing volume of air which must be further treated before it is ecologically safe for discharge. In addition, there is an obvious loss of the sometimes precious treating material itself--unless it is somehow recaptured and recycled which in itself involves yet further additional expense, effort, etc.
Accordingly, by applying only the needed amount of liquid "add-on" treatment to a fabric, there is considerable economic advantage to be had.
At the same time, in many applications (e.g., textile dyeing operations), the treating liquid must be uniformly distributed throughout the treated substrate if one is to achieve a commercially acceptable product. Furthermore, in typical commercial environments, it will be necessary for a single apparatus to successfully treat a wide variety of different types of textile substrates each having different requirements if one is to achieve uniformity.
For example, for solid shade dyeing in textile applications, the fluid jet applicator must be able to apply fluid in a uniform fashion to an entire range of commercial fabrics. Different styles of fabric vary considerably in terms of fiber content, construction, weave and preparation. These general parameters, when combined, in turn determine relative physical properties and characteristics of a given fabric such as porosity, weight, wettability, capillary diffusion (wicking) and the like. As will be appreciated, the volume of fluid per unit surface area required to adequately treat a given fabric is greatly influenced by these physical properties.
Fluid jet electrostatic applicators, such as the applicator described, in the aforementioned U.S. Pat. No. 4,650,694 are designed such that fluid is delivered out of the orifice array with a very limited operating fluid pressure range. Although the specific pressure range in a given applicator may vary depending on the size of the orifices in the array, the fluid pressure for such an applicator may, for example, be in a range of 3.5 to 4.5 p.s.i.
In such electrostatic applicators, once an optimum fluid pressure for a particular orifice array is determined, the fluid pressure level is maintained at this pressure level. In this manner, a breakup length for the droplets is provided which insures that the droplet breakup occurs while the droplet is directly opposed to the charging electrode which is on the order of 0.375 inches in length.
If the fluid pressure of an electrostatic applicator is increased to a level exceeding the above-mentioned optimum pressure, the droplet breakup length will be longer and the droplets will break up outside the charging area and will therefore, not be properly charged. Accordingly, in such electrostatic applicators, the conventional wisdom is that the maximum amount of fluid which may be placed on a substrate is limited to the volume of fluid which can be dispensed at the maximum fluid pressure for which droplet breakup would occur in the charging region.
Turning to a specific example, if a particular fabric weighs 5 oz. per square yard, it would typically require, in order to dye the fabric to a solid shade, 50% wet add-on to achieve the desired uniformity of solid shade. Given the maximum fixed fluid pressure of, for example, 4 p.s.i. to uniformly cover the 5 oz. per square yard of fabric with the 50% wet add-on requirement, the maximum speed for moving the substrate may for example, be 50 yards per minute. References herein to maintaining uniform coverage should be understood to include maintenance of a selected wet add-on level.
At this rate, the maximum amount of fluid is required out of the jet applicator to uniformly cover the fabric. This is called the "full flow" condition and is the practical limit of speed for a particular fluid applicator which is operating at a fixed fluid pressure to uniformly cover a particular fabric. When the "full flow" condition is reached all of the fluid being delivered through the orifice plate at a fixed fluid pressure is required by the substrate to maintain uniform coverage; therefore no droplets are being charged and deflected to the catcher. Prior to the present invention, operation approaching the "full flow" condition triggered the generation of a warning signal. Thus, prior to the present invention, the normal operation of electrostatic applicators has been at or below the full flow condition.
In the prior art, when operating below this "full flow" condition in the electrostatic control mode, since fluid is charged and deflected, if a jet goes out of alignment and fluid is sprayed on an electrode, a short may result. If a short develops, it will cause a defect in the fabric and will result in a dark mark or line on the fabric.
Advantageously, the method and apparatus of the present invention eliminate the possibility of shorts while operating at or beyond the "full flow" condition.
Additionally, the method and apparatus of the present invention serve to significantly increase production rates of electrostatic applicators by operating the jet applicator in an overdrive mode at fluid pressures heretofore not thought possible in electrostatic fluid applicators.
Furthermore, the present invention allows electrostatic fluid jet applicators to operate at fluid flow rates heretofore thought to be beyond the capabilities of electrostatic applicators, while at the same time maintaining a high degree of shade uniformity across the width of the fabric.
A hybrid fluid jet apparatus is disclosed whihh is particularly useful in uniformly applying liquid dye to a fabric substrate. The applicator is controlled in an electrostatic control mode while operating below the practical limit of speed for electrostatic operation to achieve uniform fabric coverage. When fluid is being supplied to the substrate at its maximum flow rate in the electrostatic control mode, the applicator senses that "full flow" condition has been reached. The applicator is then controlled to operate in a non-electrostatic control mode to control the fluid flow rate by modulating the fluid pressure received at the orifice array in accordance with the required fluid flow rate needed to achieve a uniform application of fluid to the substrate.